1. Field of Invention
The invention relates to a method and a device for the capacitive detection of flaws in polymer tubes, primarily in tubes made of crosslinked polyethylene (PE-X).
2. Related Art
Crosslinked PE-X tubes are mainly used in the building sector, where they are advantageously used as tubes for drinking water, heating, or gas.
Of the three standard methods used for the crosslinking of PE, namely with the aid of energy-rich radiation (PE-Xc), or by adding peroxide (PE-Xa), or through grafting with the aid of trimethoxyvinylsilane (PE-Xb), the yearly tonnage share of the peroxide-crosslinked tubes (PE-Xa) amounts to approximately 31% of the total production of PE-X.
To this day, the crosslinking method using peroxide above all has the disadvantage that the chemical conversion of the peroxide with the PE inside the extruder takes place quantitatively in a very narrow area that is difficult to monitor with process control and thus makes it difficult to monitor the forming of flaws, such as the forming of primary crosslinking products or the bubble-forming. Flaws of this type can lead to premature failure of the tube. For example, serious structural damages can be expected in the case of a sub-floor heating system and, if the tubes are used for gas, can even result in danger to body and life.
Attempts have thus been made from the start to optimize in particular the process technology for producing the tube and, at the same time, to develop reliable methods for detecting flaws in the extruded tube.
The following methods are considered state of the art nowadays for detecting flaws in PE-X tubes:
The High Voltage Method
With the high voltage method, the tube to be tested is pulled through a ring electrode (e.g. a ring-shaped brush electrode), wherein an inner electrode that is connected to ground is positioned at the same location on the inside of the tube. The test voltage is applied between the ring electrode and the inner electrode. The disadvantage of the high-voltage method is that it has a high danger potential because of the applied voltage in the range of approximately 12 to 50 kV and the development of ozone in cases of high-voltage breakdowns, that it depends on the tube wall thickness (outside diameter ≦20 mm) and, above all, the fact that it can detect only specific flaws (hairline cracks, through holes, and foreign matter, including primary crosslinking products). A further restriction is that the flaws must extend over nearly the complete tube wall. The high voltage method in the final analysis is designed to test the electrical insulation properties of a tube.The Ultrasonic MethodWith this method, a distinction must be made between different systems, wherein the system with ultrasonic test heads arranged stationary around a tube respectively can cover only over an extremely small, linear area, while the system with rotating test heads spirally measures and tests the plastic tube. The main disadvantage of the ultrasonic method is the extremely high investment cost, in most cases >100,000 EUR, and the fact that the method is furthermore used primarily for determining the wall thickness and/or the diameter with the aid of a transit-time measurement.
Capacitive two-electrode sensors have recently been used as sensors for detecting labels, in particular transparent labels on transparent substrates.